Method of making a waveguide window



T. J. KILDUFF ETAL METHOD OF MAKING A WAVEGUIDE WINDOW Original Filed April 9, 1959 Aug. 17, 1965 l/ I) Q G lfb L3 /5 INVENTORS ATTORNEYS United States Patent 3,201,296 METHGD OF MAKING A WAVEGUED'E WINDQW Timothy J. Kildulf, Mount Rainier, and Asaf A. Benderly, Potomac, Md assignors to the United States of America as represented by the Secretary of the Army Original application Apr. 9, 1959, Ser. No. 805,350, new Patent No. 3,095,550, dated June 25, 1963. Divided and this application Mar. 5, 1963, Ser. No. 263,359 7 Claims. (Cl. 156-108) (Granted under Title 35, US. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment to us of any royalty thereon.

This application is a division of application Serial No. 805,350, filed on April 9, 1959, now Patent No. 3,095,550 for Polytetrafiuoroethylene Waveguide Window Construction.

This invention relates to waveguides and more particularly to a method of constructing a polytetrafiuoroethylene waveguide window.

In a variety of applications it is necessary to close or seal a microwave system to retain air pressure within it and to prevent the entry of water, water vapor, dust or other extraneous matter. A sealed or pressure-resistant window is generally mounted across a section of waveguide and must be essentially transparent to microwave energy over desired frequency and temperature ranges.

Commercial waveguide pressure-resistant windows are currently constructed of electrical grade glass fused to a metal frame, or of mica sealed to a metal frame with a low-melting glass or solder. Apart from their substantial cost, these windows present the difficult problem of soldering or attachment to aluminum or brass Waveguides, and also offer some risk of corrosion due to the proximity of dissimilar metals.

One of the objects of this invention, therefore, is to provide a method of making an inexpensive waveguide window which is essentially transparent to microwave energy, which is capable of operating at high temperatures, and which is able to withstand severe shock and vibration.

Another object of the instant invention is to provide a method for bonding a Waveguide window of polytetrafluoroethylene across a waveguide section.

A further object of this invention is to provide a method which utilizes an improved non-corrosive and high tensile strength adhesive and a film of polytetrafluoroethylene for making a waveguide window.

Still a further object of the present invention is to provide a method of manufacturing a waveguide window employing a technique for bonding a film of polytetrafiuoroethylene to a metalilc waveguide section which results in an economical, airtight waveguide construction having a very low voltage standing wave ratio (VSWR).

According to this invention, a polytetrafiuoroethylene film is treated in a sodium-naphthalene bath to render it cementable and is then bonded to aluminum or brass waveguide flanges with a high-temporature-resistant adhesive containing epoxy resin and solvent. The film so bonded provides a waveguide window. Windows so assembled have successfully withstood pressures of about 20 p.s.i.g. at a temperature of 125 C. (257 F.) for 15 minutes and have provided a voltage standing wave ratio (VSWR) no greater than 1.08 when measured within a particular test-frequency band.

The specific nature of the invention, as well as other objects, uses and advantages thereof, will clearly appear from the following description and from the accompanying drawing, in which:

FIGURE 1 shows the waveguide windows of this invention prior to assembly.

3,201,295 Patented Aug. 1?, 1565 FIGURE 2 shows the assembled waveguide window.

As shown in FIGURE 1, waveguide section 10 is provided with the conventional flange 11 and a rectangular recess 12 formed in flange 11. The waveguide section 10 has the usual rectangular bore 10a extending longitudinally therethrough which is used to guide the microwave signals. The rectangular polytetralluoroethylene window 13 is cemented by means of an'epoxy adhesive 14 in recess 12. Rectangular insert 15 is insertable in recess 12 and is cemented to the edges of window 13 by epoxy-solvent adhesive 14. Flange 11 and insert 15 are preferably composed of brass or aluminum.

Polytetrafiuoroethylene window 13 can be between 0.0050.010 inch in thickness and should be devoid of pinholes. Windows having a thickness of 0.005 inch are preferred. Before window 13 is ready for bonding in recess 12 it is immersed for 15 minutes in a bath containing a one molar complex of sodium-naphthalene in tetrahydrofuran. The window is removed from the bath, rinsed first with acetone and then with water and gently wiped dry with a soft tissue. It is then ready to receive the epoxy adhesive 14 compounded from the following ingredients:

Constituent: Parts by weight Epoxy resin (viscosity 5-9 poises) 25 Chlorendic anhydride 25 Acetone 30 Methyl ethyl ketone 20 An epoxy resin which has been found to be very satisfactory is Epon 815. Epon 815 is a reaction product of epichlorohydrin and bisphenol-A, with ten percent of butyl glycidyl ether diluent. Epon 815 has a viscosity of between 6-9 poises and an epoxy equivalent of approximately 175.

Adhesive 14- is of sufiiciently low viscosity to allow extremely thin layers, less than 0.0005 inch in thickness, to be painted on the window 13. Because of the water-like consistency of the adhesive layer it has no significant effect on the electrical behavior of the window. Therefore, when the entire window is painted with adhesive it is not necessary to mask the transmission area of the window.

In addition, this thin layer provides a strong high-temperature bond for bonding the window 13 to the recess 12 as well for bonding the brass or aluminum insert 15 to the brass or aluminum flange 11.

. In assembling a typical waveguide window in accordance with this invention, a thin layer of adhesive 14 of approximately 0.0005 inch in thickness is painted over the entire side 13:: of window 13. Window 13 consists of a rectangular film of polytetrafluoroethylene 0.005 inch in thickness. Window 13 is dropped in place in the brass or aluminum recess 12, adhesive side facing rectangular bore 10a. Additional adhesive 14 is applied to the edges of the aluminum or brass insert 15 and to the face of the insert 15 which is to be pressed against side 13!: of window 13. Thus insert 15 will bond to side 1312 of window 13 and to recess 12.

If desired, the window and insert which are coated with adhesive 14 can be dried at room temperature for 15 minutes and then at 65 degrees centigrade in a forceddraft oven for one hour. The parts will then be dry to touch and can be stored for many days prior to the assembly and final cure.

After the window 13 is placed in recess: 12 and insert 15 placed against the face 131: of the window, 15 to 20 psi. pressure is applied to the insert 15 by a spring or other suitable means which can be devised by those skilled in the art. Curing of the adhesive 14 under pressure of 15 to 20 psi. is elfected at a temperature of degrees centigrade in a forced-draft oven for 3 hours.

A cured assembly such as the one described above will confine an air pressure of 20 psig. in rectangular bore a when the section 10 is heated to a temperature of 125 degrees Centigrade for minutes. The insertion of the 0.005 inch-thick polytetrafiuoroethylene window across the rectangular bore 10:: does not increase the voltage standing wave ratio (VSWR) beyond 1.08 when measured within a band consisting of 200 megacycles on either side of a particular microwave frequency. Tests indicate that the VSWR varies insignificantly between frequencies of 8,000 and 10,000 megacycles. The value of 1.08 can be compared with values of VSWR in excess of 1.10 which are typical of the substantially more expensive commercial glass or mica windows.

In light of the foregoing it will be evident to those in the art that waveguide windows composed of polytetrafiuoroethylene and constructed as described have advantages not attainable by known windows. In addition, an epoxysolvent adhesive is disclosed which has an extremely low viscosity and which is resistant to high temperatures resulting from heat developed by the flight of the missile in which the waveguides are incorporated.

It will be apparent that the embodiments shown are only exemplary and that various modifications can be made in construction and arrangement within the scope of the invention as defined in the appended claims.

We claim as our invention:

1. A method of making a window for a metallic waveguide structure comprising the steps of:

(a) chemically treating the surface of a thin film of polytetrafluoroethylene to obtain a cementable film,

(b) coating said treated surface over its entire area with a thin-layer of a low viscosity epoxy adhesive,

(c) placing said adhesively coated surface of said film against an open end of a section of said waveguide thereby closing said open section, and

(d) curing said adhesive under heat and pressure to provide an effective seal between said waveguide section and said polytetrafiuoroethylene film.

2. The method of making a waveguide window according to claim 1 wherein,

(a) the surface of said polytetrafiuoroethylene film is chemically treated with a solution containing a one molar complex of sodium-naphthalene in tetrahydrofuran to obtain a cementable film, and

(b) said treated surface being coated over its entire area with a thin layer of a low viscosity adhesive comprising an epoxy resin, a curing agent and a mixture of solvents.

3. The method according to claim 2 wherein,

(a) said epoxy resin consists essentially of a reaction product of epichlorohydrin and bisphenol-A, with ten percent of butyl ether diluent,

(b) said curing agent is chlorendic anhydride, and

(c) said solvents are acetone and methyl ethyl ketone.

4. In the fabrication of a waveguide assembly having a metallic waveguide section with a bore therethrough substantially coaxial to the longitudinal axis of said section, a flange formed on one end of said section, a recess formed in said flange, and a metallic insert for said recess having a bore therethrough to match said bore in said section, the method of constructing a window for said waveguide section comprising the steps of:

(a) chemically treating the surface of a thin film of polytetrafiuoroethylene to obtain a cementable film,

(b) applying a thin layer of a low viscosity epoxy adhesive composition to the entire treated surface of one side of said film covering said side completely,

(0) placing said film in said recess with said side of said film having said adhesive thereon facing said bore,

(d) coating the exterior edges and one face of said insert with said adhesive,

(e) positioning said insert in said recess with said adhesively coated face adjacent the other side of said film, and then (f) curing said adhesive under pressure at an elevated temperature,

(g) whereby after curing, said film provides a sealed window for said section that is essentially transparent to microwave energy.

5. The method according to claim 4 further comprising the steps of:

(a) treating said surfaces of said film by immersing said film in a bath containing a one molar complex of sodium-naphthalene in tetrahydrofuran thereby obtaining a cementable film, and

(b) applying a thin layer of a low viscosity adhesive composition consisting essentially of an epoxy resin, a curing agent and a mixture of solvents to the treated surface of one side of said film covering the entire side thereof.

6. The method as recited in claim 5 wherein,

(a) said epoxy resin is a reaction product of epichlorohydrin and bisphenol-A with ten percent of butyl glycidyl ether diluent,

(b) said curing agent is chlorendic anhydride, and

(c) said solvents are acetone and methyl ethyl ketone.

7. The method according to claim 6 wherein,

(a) said thin film of polytetrafluoroethylene is approximately 0.005 inch in thickness, and

(b) said thin layer of adhesive covering said one entire side of said film is approximately 0.0005 inch in thickness.

References Cited by the Examiner UNITED STATES PATENTS 2,527,765 10/50 Roehrl. 2,637,776 5/53 Edson 333-98 2,809,130 10/57 Rappaport 156-315 X 2,871,144 l/59 Doban 117138.8 2,882,502 4/59 Freundlich 333-98 2,932,806 4/60 Burr 174110 FOREIGN PATENTS 209,032 6/57 Australia.

EARL M. BERGERT, Primary Examiner. 

1. A METHOD OF MAKING A WINDOW FOR A METALLIC WAVEGUIDE STRUCTURE COMPRISING THE STEPS OF (A) CHEMICALLY TREATING THE SURFACE OF A THIN FILM OF POLYTETRAFLUOROETHYLENE TO OBTAIN A CEMENTABLE FILM, (B) COATING SAID TREATING SURFACE OVER ITS ENTIRE AREA WITH A THIN-LAYER OF A LOW VISCOSITY EPOXY ADHESIVE, (C) PLACING SAID ADHESIVELY COATED SURFACE OF SAID FILM AGAINST AN OPEN END OF A SECTION OF SAID WAVEGUIDE THEREBY CLOSING SAID OPEN SECTION, AND (D) CURING SAID ADHESIVE UNDER HEAT AND PRESSURE TO PROVIDE AN EFFECTIVE SEAL BETWEEN SAID WAVEGUIDE-SECTION AND SAID POLYTETRAFLUOROETHYLENE FILM. 